Primary pollutants (SOx, NOx, hydrocarbons) are emitted directly into the atmosphere and are found there in the form in which they were emitted. The can come from natural and human-made sources. They are substances directly produced by a process, for example, ash from a volcanic eruption, carbon monoxide from motor vehicle exhaust, and methane from plant decay.
Primary pollutants produced by human activity include:
- oxides of sulfur, nitrogen and carbon
- organic compounds, such as hydrocarbons (fuel vapour and solvents)
- particulate matters, such as smoke and dust
- metal oxides, especially those of lead, cadmium, copper and iron
- odours (from chemical such as acetone)
- toxic substances, for example, 1,1,1-trichloroethane, acetone
Often, primary pollutants react with water, with oxygen in the air, or with one another, and spawn a whole new set of pollutants. These pollutants can be called secondary pollutants, as they are produced from the chemical reactions of primary pollutants. These reactions are often triggered and powered by energy from the sun; for example, tropospheric ozone is one of the secondary pollutants that makes up photochemical smog. Secondary pollutants may include particulates formed from primary pollutants, and compounds in photochemical smog, such as nitrogen dioxide, ozone and peroxyacetyl nitrate (PAN).
Note that some pollutants may be both primary and secondary; that is, they can be emitted directly from a process, and they can be formed from other primary pollutants. For example, ozone is a secondary pollutant when it is formed in a photochemical process in the presence of NOx and hydrocarbons. Ozone can also be formed as a result of lightning — a natural event — or generated from the printers and copiers in our offices — human-made sources.
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